After the introduction of the first EDI systems in the late 1980s EDI was rapidly commercialized first for the production of high purity water in industrial processes, then for the deionization of non-ionic streams in industrial processes (e.g. urea desalting), and more recently for the production of high purity water in laboratory systems. The reason for the rapid adoption of EDI processes in such a wide range of applications is that EDI is among the most cost effective water purification processes for the removal of ionic contaminants to produce high purity water.
While different EDI systems operate over a water quality (WQ) range of 1˜18 MΩ cm, individual EDI systems do not provide a selectable range or a target water quality nor are capable of maintaining a constant WQ during the useful life of the EDI modules. In contrast, single-stage reverse osmosis (RO) water purification systems cannot reliably produce water with a WQ above 1 MΩ·cm, and oftentimes lower than 0.1 MΩ·cm.
EDI systems used to produce high purity water in laboratories should be simple and inexpensive while simultaneously delivering high purity water reliably with minimum or no intervention by end-users. Furthermore, most of these systems use tap water as its feed water, which has a large variety of contaminants with wide ranges in their concentration depending on the location of the installation and the season of the year. All of these factors result in a very large range of ionic contaminants presented to a water purification system, both in the form of dissolved salts and weakly ionized species such as CO2, silica and boron. As a result, the use of EDI processes for water purification systems in laboratories is among the most demanding application of EDI technology, since notwithstanding these challenges the water purification system should perform as a simple laboratory appliance.
U.S. Pat. No. 5,762,774 discloses the use of a suitable source of DC current (a variable current power supply) and a current ratio based on Faraday's Law and calculates the current applied to the EDI module based on the ionic load to the EDI module to achieve a “pre-specified level of deionization.” The current ratios disclosed are from 1 to 50 times the minimum or Faraday current. The document does not specify a range for the water quality and is silent on how to address the start-up period after an extended idle time. Similarly, U.S. Pat. No. 6,365,023 discloses the use of a constant current power supply at a current ratio which is sufficiently high to ensure that the current applied to the module accommodates the large dynamic range of ionic loads encountered in laboratory systems. The ratios claimed are from 1 to 15 times the Faraday current.
U.S. Pat. Nos. 6,391,178 and 6,726,822 disclose the intermittent application of DC power to an EDI module to obtain WQ in a specified range of water quality. The documents disclose the use of either constant current or constant voltage power supplies to deliver water in a range of water quality by adjusting the amount of time the power supply is ON and OFF. The preferred proportional band control scheme described will not work as intended as it will either undershoot or overshoot the lower and upper bounds, respectively, of the specified range of water quality depending on how much current the power supply is delivering relative to the current required to maintain the WQ within the range. As such, the control scheme disclosed will lead to a constant cycling of the WQ above or below the specified range of water quality. Additionally, since sufficiently high currents need to be supplied to accommodate the highest possible ionic loads, for most installations a current much higher than necessary is applied during the part of the cycle when the power supply is ON, which increases the tendency to scaling within the EDI module. Moreover, the documents also do not disclose a method for achieving a constant WQ, and are silent on how to address the start-up period after an extended idle time.
U.S. Pat. No. 6,607,668 discloses an EDI-based water purification system utilizing RO pretreatment that further comprises a control system that monitors the WQ and “calculates the required electrical voltage and current required by the EDI module and automatically adjusts each to achieve optimum outlet water quality.” It is unclear what adjusting “each” means since the electrical voltage and current cannot be adjusted individually. The document does not describe the method by which the control system calculates the required electrical voltage and current, and also does not set a range for the water quality and is silent on how to address the start-up period after an extended idle time.
Japanese Pat. No. 4,954,926 discloses a fuel cell system that reuses the condensation water to feed the reformer in the fuel cell, an EDI module to deionize the condensation water, and a variable voltage power source to drive the EDI module. In one embodiment disclosed in the document the conductivity of the deionized water is monitored and the voltage of the power source is varied to obtain a deionized water conductivity above a predefined threshold value. Another embodiment refers to the use of an ammeter to measure the current to the EDI module and adjusting the voltage of the power source to obtain a current above a threshold value defined beforehand. The preferred embodiment uses the two voltage levels supplied by the fuel cell itself—12 and 24 V—as the variable voltage power source obviating the need for a separate power supply. There is no hint to set a target and/or range for the water quality or how to address the start-up period after an extended idle time.
A conventional approach to control an EDI module in industrial processes is to drive it at constant voltage. These systems are custom-designed based on the feed water present in each installation, enabling specialized pretreatment and tailoring the size and configuration of the EDI module to the ionic load. Furthermore, these systems need to be professionally maintained ensuring that the feed water to the EDI module is constant. Finally, in these installations the EDI module can be cleaned to remove contaminants bound to the EDI module that reduce its effectiveness. With these measures the electrical impedance of the EDI module remains approximately constant, enabling a constant voltage power source to deliver an approximately constant current sufficient to effectively remove to the ionic load to the EDI module.
In contrast to industrial applications, where the impedance of an EDI module remains approximately constant, in laboratory applications the impedance may be different in each installation and gradually increases for all the reasons already stated: the tap water is different in different installations; a single standard system design needs to perform reliably in all installations; the module is not cleaned to remove “foulants.” A module driven at constant voltage will initially be driven with a high current because it has a low impedance. As the module ages its impedance may increase with a proportional drop in current, eventually leading to insufficient current in some applications, and low water quality. As a result, the state of the art of EDI-based laboratory systems is to drive the EDI module with a constant current power source. These systems initially deliver very high WQ, often exceeding 17 MΩ-cm, but as the modules age the WQ gradually drops, eventually reaching very low values unacceptable to end-users, at which moment the module is replaced. Furthermore, since these systems are driven at the same constant current, for most installations the module is being driven with a current much higher than necessary, possibly 10 times higher, resulting in an increased tendency to scaling and a concomitant reduction in module life.
None of these conventional methods for the control of an EDI module are capable of delivering high purity water within a narrow range of water quality. Most of these conventional methods require a priori knowledge or some measurement of the ionic load to the EDI module, and these conventional methods deliver an excessive current to the EDI module increasing the risk of scaling and increasing the power consumption. Furthermore, none of these methods address the challenges unique to laboratory water systems: having a changing ionic load and being characterized by an intermittent operation; requiring a system that produces the target water quality day-in, day-out, and rapidly recovering the water quality after an extended idle period. In addition to the inability to deliver purified water within a user-specified range of water quality and at a user-specified target, two common problems encountered in the practice of EDI are inadequate ionic removal which leads to poor WQ and scaling which when unattended can quickly lead to premature failure of an EDI module. Both of these factors can lead to poor output water quality.